1. Field of the Invention
The present invention relates to a method and apparatus for removing water from a slurry of particulate material, such as coal, and more particularly concerns an improved vibratory screen apparatus for conveying the coal.
2. Description of the Prior Art
Dewatering is the extraction of surface moisture from particulate coal, where surface moisture is water retained on the surface of a coal particle as contrasted to inherent moisture which is retained within the pores of the coal particle. Modern mining techniques produce a higher proportion of fine coal particles which have a higher surface area to weight ratio than coarser particles produced by earlier techniques. Such fine coal particles are more difficult to dewater than coarse coal particles.
Vibratory screen separators have been used for many years to dewater coal. Such separators typically include a shallow trough having vertical sidewalls and a flat bottom formed of a heavy duty screen, such as a rod screen having a size in the range from 1/4 mm to 1 mm openings. The trough is mounted on coil or leaf springs, known as isolator springs, and a vibratory drive unit, such as one including eccentric weights, is attached to the trough so as to induce oscillations along a predetermined axis. A layer of coal several inches thick is conveyed across the screen by the oscillations, in the well-known manner of vibratory conveyors, and is discharged over the end. The vibrating action helps dewater the coal, but because of the water retention characteristics of particulate coal, it is difficult to lower the surface moisture content below 20% based on the dry weight of the coal. An example of such a vibrating screen separator is the model SF-66 vibrating screen feeder manufactured by the FMC corporation, Homer City, Pennsylvania and described in Bulletin 220572 (pp. 26-33). A similar separator is described in U.S. Pat. No. 2,457,018 to Wantling.
The phenomenon of water retention in beds of particulate coal has been described by C. C. Harris and H. G. Smith in a published paper presented to the Second Symposium of Coal Preparation at the University of Leeds (England), Department of Mining, Oct. 21-25, 1957. The authors identified four distinct modes of water retention. The first such mode is denominated free moisture. Free moisture is that water which will freely drain from the coal as soon as a drainage path is available. Thus, free moisture is not "retained" by the coal at all. The remaining three modes described by Harris and Smith act to retain moisture in the interstices between adjacent particles even when the water is otherwise free to drain.
The second of the modes described by Harris and Smith is the capillary mode wherein water is held in the narrow passageways between particles by capillary action. The amount of water retained by capillary action is limited and depends on both the force pulling down on the water (normally gravity) and on the average diameter of the capillary-like passages. For a true capillary having a circular diameter d at its top, the height h of the liquid column is: ##EQU1## where, h=height of liquid column
.nu.=surface tension of liquid-solid interface PA1 .theta.=contact angle of liquid solid interface PA1 d=diameter of capillary tube at top PA1 l=liquid density PA1 g=gravitational constant
The third mode described is the funicular mode. Without further describing this mode, it is noted that the water held in this mode is released when the water held in the capillary mode surrounding it is released.
The final mode described is the pendular mode. In this mode, water is held in discrete rings surrounding the contact points between adjacent particles. To release water retained in this mode it is necessary to break contact between the adjacent particles. Harris and Smith estimate that approximately 20 percent of the retained surface moisture (i.e., surface moisture other than free moisture) is held in the pendular mode, while the remaining 80 percent is held in either the capillary or funicular mode.
The prior art vibrating screen separator described hereinbefore removes only free water from the wet coal feed and the product stream therefrom still contains the water retained in the remaining three modes just described. To remove such retained water, additional means must be used. One approach commonly employed is to provide a vacuum chamber beneath the vibrating screen to draw out the retained water. Such a vacuum dewatering system is disclosed in U.S. Pat. No. 3,929,642 to Ennis et al. An alternate approach is to provide a centrifuge downstream of the vibrating screen.